Poor Execution
For a game that's abbreviated as STD, I wanted to like this game. I also love tower defense games and DDR, which were both heavy influences for this game (the swirl selector and pentagraph are obviously taken from it), so I was really frustrated when I found that this just didn't appeal to me. Although the graphics are very nicely done and look natural, and the YouTube video streamed from this is high quality and transitions nice within the Flash canvas, AND you made a pretty decent music selection, I can't honestly say I enjoyed it. Why? Because the game isn't involved enough!
For one, it seems that which notes in a song you decided to turn into actual notes on the staff were chosen at random. Plenty notes were in the songs and much fewer were shown as enemies. Perhaps you could have made the game a little more accurate by giving the levels more notes with fewer health. Also, it doesn't even seem as if the notes go with the music! Perhaps a tile-based system would have made the layout appear more segmented, but as it is I don't even see how it's that different from most tower defenses. Last, I hate how you unlock things at such a slow pace! It gives the player less options, makes it more frustrating, and in general detracts from the gameplay.
Also the fact that you TELL players which towers to use makes it much less appealing as well. The special attacks and other doodads didn't seem to impact the gameplay much and were trivial, tacked on features.
It was definitely a good idea, but right now it still looks pretty sketchy and not fun at all.